This invention relates to dry, extruded cereals formulated with sweeteners and selected high amylose starches.
Native high amylose starches are corn starches from hybrid varieties of corn which contain at least about 40% amylose. In contrast, ordinary corn starch typically contains about 28% amylose.
The use of granular non-pregelatinized high amylose starch, optionally modified, in farinaceous-based extruded products is disclosed in U.S. Pat. No. 3,407,070 (issued Oct. 22, 1968 to D. G. Murray et al.). The suggested food products include crackers, chips, cereal puffs, and other "snack" items. The total concentration from the high amylose starch is from about 5 to 60%, based on the total weight of solids in the blend. Concentrations below 5% result in gummy extruded products of low rigidity. Concentrations exceeding 60% tend to make the extruded product exceedingly dense thereby precluding the attainment of a porous, puffy appearance. Moisture should be present in the extruded mixture. Water concentrations from 15-50%, preferably 20-40%, based on the total weight of the composition, are ordinarily used. The moisture inherently present in the various components is not included in determining the amount of water to be added to the dry blend. Sufficient heat and pressure are used to hydrate and gelatinize the starch and cook the farinaceous base. Typically a barrel temperature of about 200.degree. to 350.degree. F. and a pressure of about 50 to 1000 p.s.i. is used. Where the product emerges from a hot extruded die and flashing off is used to put the product in its final form, there is no need for a subsequent cooking step. Where the product emerges from a cooled extruded die, the product is cooked further (e.g., baked or fried) to reduce the moisture content.
The relationship between the amylose content and the extrusion-expansion properties of corn starches containing 10, 25, 50 and 65% amylose was studied by R. Chinnaswamy et al. (Cereal Chemistry 65, No. 2, 138-143 (1988). Granular starches and starch mixtures were extrusion cooked at different temperatures and different moisture contents. No other ingredients were present to compete with the granular starch for the water. All starches were extruded under optimal conditions. The expansion ratio initially increased from 8.3 to 16.4 as the amylose content of the native starch increased from 0 to 50% and then decreased sharply. The maximum expansion ratios were 11.9 for 0% amylose (i.e., waxy starch), 14.2 for 25% native amylose, 16.4 for 50% native amylose, and 11.8 for 70% native amylose which occurred at 130, 140, 150, and 160.degree. C., respectively. Blended starches (obtained by blending native corn starches with native waxy starch (which contains no amylose) showed similar trends but with lower expansion. The results show native starches having 50% amylose levels expanded best and that starch type, in addition, to amylose content plays an important role in determining expansion characteristics during extrusion cooking.
Due to the highly bonded linear structure of high amylose starches, full and effective gelatinization of granular high amylose starches typically requires cooking temperatures of about 154-171.degree. C. However, in the Chinnaswamy et al. study, even at 160.degree. C. the native 70% amylose starch was no better than the waxy starch.
Ready-to-eat breakfast cereals generally fall into one or two categories, i.e., non-presweetened and presweetened cereals. In spite of this industry-recognized distinction, both products contain sweeteners which provide many functional properties in addition to flavor. Non-presweetened cereals usually contain 6-12% sucrose, whereas presweetened cereals usually contain 6-20% sucrose. Sweeteners not only affect the finished cereal, they also may dramatically influence the cereal's behavior during production, particularly with extruded cereals where the presence of high sweetener levels can even determine equipment design.
As is pointed out in the article "Sugar in the Mornin':Sweeteners in Breakfast Cereals", sweeteners affect the chemical and physical properties of extruded cereals because they compete with starch for the available water. Certain effects are generally observed. First and foremost is inhibition of starch gelatinization which alters the cooking requirements for the cereal mass and reduces the viscosity of the mass. By holding onto moisture, sweeteners also inhibit moisture release which hampers the product's expansion. To assure proper starch gelatinization, the residence time and/or process temperature can be increased or the screw design can be adjusted.
Sweeteners will also affect the cooking of whole grains for flaking, puffing, or shredding, because, as with extrusion, there is competition for moisture which creates the need for increased processing temperature or extended cooking times to gelatinize the starch. In addition, the sweeteners can make the grain mass sticky and difficult to machine. According to the above article, this is particularly noticeable in flaking and puffing where individual grain identity is a requirement. Few processing solutions exist and the stickiness is usually controlled by monitoring the overall sugar level.
Thus, at the relatively low total processing moisture content used in extruding the cereals, i.e., 15-22% for extruded puffed cereals and 25-35% for extruded flaked cereals, and at the typical extrusion temperatures of about 120-160.degree. C., granular high amylose starches will not gelatinize in the presence of sweeteners.
High amylose starches pregelatinized by conventional methods are not suitable because conventional methods such as drum drying or jet-cooking and spray-drying produce retrograded starches, or starches that are incapable of full dispersion, or starches that are so degraded that their functional benefits are substantially reduced. See U.S. Pat. No. 3,515,509 (issued Jun. 2, 1970 to Feldman et al.) which discloses that drum-dried potato amylose retrograded upon storage and was unsuitable in package foods. See also the discussion in U.S. Pat. No. 5,188,674 (issued Feb. 23, 1993 to J. Kasica et al.) regarding the difficulty of preparing soluble high amylose starches by jet cooking and spray drying because the starch cooks are difficult to atomize because of their high viscosity and because the starch retrogrades.
The gelatinization and dispersibility characteristics of high amylose starches can be modified by derivatization. The use of these "modified starches" is undesirable in food products that are advertised as "natural" products. In addition, the gel strength and water-resistance of "modified" starches are also reduced in proportion to the amount of derivatization.
Thus, there is a need for soluble high amylose starches that can be formulated into ready-to-eat, sweetened extruded cereals without altering the conventional processing conditions and still provide puffed or flaked cereals with acceptable eating characteristics.